1. Field of the Invention
This invention relates generally to an apparatus for continuously curing a heat curable polymeric covering on an elongated conductor and particularly to such an apparatus of the catenary type.
2. Prior Art
One conventional vulcanizer of the catenary type, designated generally at 10 in FIG. 1, comprises metering catapillars 11 for feeding a conductor 12 in one direction at a constant speed, an extruder 13 for applying a covering material on the conductor 12 fed from the metering catapillars 11, a vulcanizing tube 14 for passing the cable 12a therethrough, and tensioning catapillars 15 for hauling the cable 12a. The vulcanizing tube 14 has a curing section 14a arranged in the form of a catenary and extending from its inlet 14b to a location intermediate its inlet 14b and outlet 14c. The curing section 14a is filled with a hot curing medium under pressure. Thus, the covering material on the conductor 12 is cured during the passage of the cable 12a through the curing section 14a. A positioning sensor 16 is provided in the curing section 14a of the vulcanizing tube 14 for sensing the position of the cable 12a in the curing section 14a. A signal is fed from the positioning sensor 16 to the tensioning catapillars 15 through a control circuit so that the speed of rotation of the tensioning catapillars 15 is electrically controlled to impat a required tension to the cable 12a, thereby maintaining the cable 12a in the form of a catenary in the curing section 14a. Therefore, the cable 12a is always kept away from the internal wall of the vulcanizing tube so that the uncured covering material is prevented from being deformed. The covering material on the conductor 12 is cured during the passage thereof through the vulcanizing tube 14. The cable 12a is moved in sliding contact with the bottom of the internal wall of the vulcanizing tube 14 after it passes a touch down point 17. The covering material has been completely cured before the cable 12a reaches the touch down point 17. The vulcanizing tube 14 also comprises a cooling section 14d disposed forwardly of the curing section 14a, a cooling medium being filled in the cooling section for cooling the cable 12a passing therethrough. An end seal box 18 is attached to the outlet 14 c of the vulcanizing tube 14 to protect against the leakage of a medium. The cable 12a passes through the end seal box 18 and gripped by the opposed tensioning catapillars 15.
The catenary of the cable 12a is defined by the following formula: EQU C(cm)=T.sub.L /W
Wherein C is a catenary factor, W(Kg/cm) is a unit weight of the cable 12a, and T.sub.L is a lower (horizontal) tension.
Also, the catenary of the cable 12a is indicated in coordinates in FIG. 2, and the following formula is obtained: EQU Y=C cos hx/c
Therefore, in order to make the catenary factor C constant (for example, the distance between the cable 12a and the internal wall of the vulcanizing tube 14), the lower tension T.sub.L and an upper tension T.sub.H must be made constant because the unit weight W is constant.
As described above, the covering material on the conductor 12 is completely cured at a location adjacent to the touch down point 17, so that the solidified covering 12b is formed on the conductor 12 in concentric relation thereto, as shown in FIG. 3. A pressure P(kg/cm.sup.2) of the hot curing medium in the vulcanizing tube 14 is exerted on the covering 12b of the cable 12 so that a tension T.sub.2 (kg) is applied to the cable 12a in the direction of movement thereof.
The tension T.sub.2 is obtained from the following formula: EQU T.sub.2 =.pi./4D.sup.2 P
Wherein D is an outer diameter of the cable 12a.
In the case where the curing medium has a relatively large specific gravity, the curing medium imparts a considerable buoyancy to the cable 12a. As a result, the apparent weight W of the cable 12a becomes lighter, and the upper tension T.sub.H and the lower tension T.sub.L has to become less because the catenary factor C is constant in the vulcanizing tube 14. T.sub.L is obtained from the following formula: EQU T.sub.L =T.sub.1 +T.sub.2
The tension T.sub.1 can be reduced by controlling the speed of rotation of the tensioning catapillers 15a, 15a. As described above, the tension T.sub.2 is determined by the pressure P exerted by the pressurized curing medium on the cable 12a. Therefore, if the pressure P is reduced below a predetermined level so as to reduce the tension T.sub.2 to maintain a desired catenary, the covering material on the conductor 12 is foamed because the pressure compressing the covering material is reduced, and the electrical characteristics of the cable 12a is adversely affected.
In a conventional vulcanizer of the vertical type, a tension T.sub.2 (kg), exerted on the cable 12a in the direction of movement thereof by a pressure P(kg/cm.sup.2) of a pressurized hot curing medium in a vertical vulcanizing tube, is obtained from the following formula: EQU T.sub.2 =.pi./4(D.sup.2 -d.sup.2)P
wherein d is an outer diameter of the conductor 12 and D is an outer diameter of the cable 12a. With this vulcanizer of the vertical type, the tension T.sub.2 has no effect on the position of the cable in the vulcanizing tube but has effect on the gripping force of the metering catapillars. Therefore, if the pressure P of the hot curing medium is increased, the gripping force of the metering catapillars must correspondingly be increased. Thus, the metering catapillars becomes bulky to meet such requirements, and the overall construction cost of the vulcanizing apparatus becomes more expensive.